Thermodynamic behaviour of acridine orange in solution. Model system for studying stacking and charge-effects on self-aggregation

Abstract

The thermodynamics of association of the positively-charged dye Acridine Orange have been investigated spectrophotometrically in water as a function of ionic strength and in the presence of added methanol, urea and dioxan. The extinction coefficients ε_M, ε_D, and ε_St, were measured which are characteristic of the spectra of monomer, dimer and long aggregates of the dye molecules respectively. A theoretical model system, based on short-range stacking and long-range electrostatic interactions, has been used to interpret the thermodynamic data. The results suggest that both factors are of importance in determining the tendency to aggregate. Furthermore, when the solvent is varied they are related by a compensation effect; decreases in stacking interactions are paralleled by decreases in repulsive electrostatic interactions.

The results of this study on a model self-associating system are of relevance to the general problem of the importance of stacking and charge effects as factors determining micelle formation, association of nucleotides and nucleosides and the conformational stability of biological macromolecules in solution.